This invention is generally related to metallic flakes used in coatings such as thin films and paints, and specifically to methods of achieving a uniform, thin, protective coating on certain types of metallic flakes using fluidized beds and fluidized bed techniques.
Metallic flakes are added to coatings such as films and paints to achieve a number of desirable effects that include enhanced engineering effects as well as improved artistic effects. One common application for the use of metallic flakes to achieve esthetic effects is in automobile paints wherein the bright flakes provide a luster or sparkle to the automobile finish. Of course, one of the problems with these paints is that the finish of the automobile must be protected with a transparent finish such as a wax or clear coat or the sparkle fades. The fading is due to the oxidation of the metallic flakes. As the flakes oxidize, the oxidized metal flakes are generally not as reflective as the unoxidized metal flakes, so the automobile finish appears to have become dull.
Other uses for metallic flake include incorporation into articles or coatings applied to articles in order to achieve electromagnetic interference suppression, infrared radiation suppression, heat reflectance from hot surfaces and as an insulator. In the latter case, the flakes can act as an electrical insulator even though the flakes are metal, if the flakes can be properly oxidized. When properly oxidized, the flakes are not conductive even in the presence of an electric field, as the oxidized surfaces prevent any continuity of a circuit. However, the oxidation of the flakes typically produces a dull surface finish which reduces the effectiveness of the film as a heat reflector or as an infrared radiation suppressor. In addition to the surface finish of the particle, the relative orientation of the particle with respect to the article surface can have an impact in determining the degree of brightness or reflection or the coating. Of course, when the oxidation treatment on the surface finish of the particles produces bright surfaces, the overall ability of the coating to provide optical reflection is also improved. The problem of relative orientation of non-spherical particles is the subject of a co-pending application assigned to the assignee of the present invention, which is identified as Ser. No. 09/489,717.
Current technology exists to oxidize equiaxed particles and spherical particles containing aluminum using oxygen in fluidized beds. This technology produces a particle in which the selective oxidation of aluminum has been controlled in a manner such that the oxidation of the particles does not adversely affect the brightness of the particles, yet provides the required oxidation resistance and corrosion resistance to preclude further deterioration of the particles while still maintaining sufficient insulative properties of the particles. But these spherical and equiaxed particles are not as effective as flake particles for esthetic uses and for engineering uses such as heat reflectance, infrared radiation suppression and electromagnetic interference suppression. However, current fluidized bed technology cannot effectively be used in a similar fashion for the selective oxidation of flake. Other available processes utilize solution coatings in combination with fluidized bed processes to form protective barriers on spherical and equiaxed particulate.
There is, however, a need for an effective method to produce metallic flake particles using a fluidized bed that are oxidized in a controlled manner so as not to unduly affect its brightness and ability to reflect radiation in the infrared and visible light spectra while still retaining properties such as the ability to act as electrical insulator.
The present invention provides metallic non-spherical particles of aluminum containing alloys in which the outer surface of the particles undergo a controlled oxidation utilizing fluidized bed techniques. As used herein, spherical particles refers both equiaxed particles and spherical particles in which the aspect ratio of the particles is about one or very close to one, while non-spherical particles refers to particles having an aspect ratio greater than one and less than about two hundred, and typically greater than ten and less than fifty and includes flake.
In one form, the present invention includes non-spherical particles made from a metallic alloy that include at least a small amount of aluminum in which the outer surface has been selectively oxidized in a controlled fashion to form a thin protective ceramic shell of alumina having good reflectance while the metallic core within the shell is substantially unaffected by the oxidizing treatment.
In another form, the present invention provides a method for selectively oxidizing the aluminum-containing alloy non-spherical particles in a controlled fashion utilizing fluidized bed techniques. The method permits the formation of a thin protective transparent alumina shell or jacket around the particles to provide environmental protection and electrical insulation capabilities, but without adversely affecting the brightness characteristics associated with metallic reflectance required for infrared radiation suppression and heat reflectance. The brightness of the oxidized particle is characterized by a reflectance of at least 60%. The method of the present invention requires the specific injection of oxidizing agents into a fluidized bed of particles suspended at an elevated temperature in a flow of reducing gas at a predetermined rate. The ratio of the weight/volume of the oxidizing agent to the weight of the non-spherical particles is predetermined to achieve the desired oxidation.
In another form of the method, the non-spherical particles are mixed with fluidizing aids to enhance the selective oxidation of the non-spherical particles without either agglomeration or spouting of the particles that can otherwise result from their shape. Spouting is a process in which particles are blown in an uncontrolled fashion from a controlled, substantially equilibrium-type suspension. The careful selection of processing aids and control of the processing parameters within the fluidized bed permits the controlled and selective oxidation of the particles without the undesirable effects of either agglomeration or spouting.
In yet another form of the method, at the completion of the oxidation of the non-spherical particles, the non-spherical metallic particles are conveniently and effectively separated from the processing aids.
An advantage of the present invention is that non-spherical metallic particles are produced using fluidized bed techniques. Thus, existing fluidized bed equipment and processes can be modified to accommodate non-spherical particles.
Another advantage of the present invention is that fluidized bed techniques can be used to selectively oxidize aluminum-containing non-spherical particles to provide particles that are resistant to environmental deterioration in corrosive and oxidative atmospheres at elevated temperatures. These particles can be oxidized to provide this transparent, environmentally resistant protective layer in a controlled fashion with minimal effects on the reflective capability of the outer surface. These particles, although oxidized, have improved optical reflection capabilities over prior art oxidized flakes, and the improved reflection capabilities extends to electromagnetic and radiation capabilities as well.
Still another advantage of the present invention is the ability to combine metallic non-spherical particles with processing aids in fluidized bed equipment in order to achieve a controlled suspension of the particles within the fluidized bed during heat treatment and oxidation in the fluidized bed. Associated with this advantage is the additional advantage of being able to separate the oxidized and heat treated non-spherical particles from the processing aids so that the particles can be used in subsequent applications with little or no additional processing.
Continuing and often interrelated improvements in processes and materials, such as the improvements of the present invention, can provide cost reductions and major increases in the performance of devices such as aircraft gas turbine engines, lamps, composites and the like.